SLC (single-level cell) flash is often the more desirable
choice for enterprise SSDs, a consequence of the margin
for error in reading a bit from a cell being very low while
the endurance remains quite high. MLC flash, on the other
hand, has occurrences of higher bit errors and suffers from the perception
that it has much lower
endurance. Efforts are being
made, however, to use MLC flash in enterprise
SSDs because they can provide more storage
and reduce the cost of production.

Through HET implementation, Intel has thought of a
way to attain MLC flash with higher reliability by exploiting
the fact that not all MLC flash chips in a batch have
the same characteristics. Some have higher read margins
and improved retention quality than others, and an understanding
of these characteristics is what makes up Intel’s
use of MLC NAND flash as HET in the Intel 710 SSD.

1. Twenty MT29F16B08CCME1 NAND MLC ICs, mirrored on the front and the back of the board, make up the storage of
the 200-Gbyte version of the Intel 710. Each NAND flash device package contains two stacked 64-Gbit, 25-nm L74A
NAND flash dice, making up 128 Gbits per package. The total flash capacity is actually 320 Gbytes, so a high level of
overprovisioning was implemented.

2. The controller uses a 512-Mbit Hynix H55S5162EFR mobile SDRAM for
system memory. Intel’s decision to
utilize a mobile SDRAM makes sense,
as it likely provides the required level
of performance while, at the same
time, minimizing power consumption.
Reducing power consumption here
is important because of the heavy
amount of traffic that the DRAM
would see while operating the drive,
especially in enterprise applications.

3. Six 470-μF capacitors connected in parallel are used to store the
required emergency charge and keep the drive alive long enough
to flush out ongoing operations in the event of a power failure. Total
capacitance is approximately 2.8 mF. This approach adequately
powers the drive for a short amount of time without compromising
large amounts of board space.

4. The SSD uses an Intel
PC29AS21BA0 controller. Other
Intel SSDs use this controller, but
it is likely that a firmware update
has made its operation unique to
the 710 drive.

The use of ONFI

An Intel 29F16B08CCME1 25-nm MLC NAND flash device was connected to our flash tester. The device determined that it is ONFI (Open NAND Flash Interface)-compliant. The detailed ONFI parameter bytes that the device returned (a detailed list of device parameters and supported capabilities) showed that it supported up to ONFI version 2.2 and a wide array of asynchronous operations for the controller to make use of.

The ONFI specification describes the option of having a separate I/O power supply VCCQ at a lower voltage of 1.8V, which Intel has implemented in the Intel 710 SSD. Thus, the I/O bus activity consumes significantly less power than if it was at the device VCC of 3.3V.

The NAND flash devices were monitored at their pins during SSD read-and-write operations to view the controller interaction with the flash. The controller had a high level of parallelism, right down to each die with multiplane operations, and maintained an organized and even distribution of the data written across all flash devices.

Intel’s innovative approach of identifying and using high-quality MLC NAND flash in its 710 SSD, coupled with the reliability during power outage, very large overprovisioning, and reduced power consumption, makes the Intel 710 a unique and innovative enterprise SSD that re-emboldens the distinction between enterprise- and consumer-grade storage.